To define the exact elements on the HIV LTR that are responsible for transactivation by HSV, we studied the effect of HSV-1 infection, or transfection with HSV-1 ICPO or ICP4, on the activation of HIV LTRs which contain mutations that delete either the SP1 binding sites, the NF-kB sites, or both NF-kB and SP1 binding sites (double mutants). Lesions in either the SP1 or NF-kB sites are not lethal to the virus.Virus with the double mutant, however, does not replicate. We studied the effect of HSV-1 infection and its immediate early genes on the activation of these HIV LTR CAT constructs in both Vero and Jurkat cells. In Vero cells, HSV-1 infection, and ICP4 or ICPO alone can activate either of the single mutants, with the SP1 deleted mutant being activated less than the wild type. The double mutant could not be activated. In Jurkat cells similar results were obtained with each HSV-1 transactivator. Surprisingly, if cells were treated with PMA, PHA, and superinfected with HSV-1, we were able to activate the expression of the double mutant. This finding led us to postulate that an additional target on the HSV LTR may respond to HSV-1 infection.